Capacitive pressure sensors of the type concerned with herein consist of a hermetically closed housing having a reference pressure therein. One side of the housing has the shape of a flexible diaphragm. On the diaphragm a capacitor electrode is located and on an opposite surface, inside the housing, another capacitor electrode is located. Both of the capacitor electrodes are connected to an electrical measuring circuit. When the pressure acting on said diaphragm varies the diaphragm is flexed. Thereby the distance between said both capacitor electrodes is varied whereby the magnitude of the capacity will vary in the capacitor formed by these two capacitor electrodes. Other locations of said electrodes are also possible.
For a pressure sensor of this type the capacitance of the capacitor electrodes is composed of a capacitance varying inversely proportionally to the distance between the capacitor electrodes and a further partial capacitance, not varying in this way and deriving, from among other things, stray capacitances and border effects in the flat capacitor formed by said electrodes. When the capacitance of the capacitor electrodes is measured in order to obtain a measure of the bending of said diaphragm and consequently of value of the pressure acting on said diaphragm this further partial capacitance is a source of errors.
Pressure sensors of this prior art type are described in e.g. U.S. Pat. No. 4 207 504, and U.S. Pat. No. 3 948 102.
To reduce said further partial capacitance and simultaneously constitute an electrical shielding of said capacitor electrodes it is previously known to provide a conducting layer on the outer surface of the pressure sensor, see for instance US A 3 858 097, EP AI 0 095 966, US A 3 750 476, though these documents do not directly describe a pressure sensor of the type set forth above. Thus the first of these documents is concerned with flat pressure sensors having two equally thick diaphragms located adjacent to one another and the third document is concerned with a pressure sensor having a cylindrical diaphragm.